1. Field of the Invention
The present invention relates to methods for operating radios for use in vehicles, and, more particularly, to methods for avoiding tuning into frequencies having poor signal quality in radios for use in vehicles.
2. Description of the Related Art
Co-channel interference in radio head units occurs when two or more stations with different audio content broadcast on the same frequency. Co-channel interference may be more likely to occur in adverse weather conditions (e.g., high pressure weather conditions) in which signals that would normally exit through the atmosphere instead get reflected back down to earth by the troposphere. This in particular applies to Amplitude Modulated (AM) signals. Co-channel interference may also occur due to poor network frequency planning by broadcasters, resulting in re-use of frequencies in a localized area.
The difficulty presented by co-channel interference is how to properly recognize the co-channel situation and avert it in a radio head unit. Aftermarket and OEM DSP chip solutions for radio head units support co-channel detection logic for only AM and not FM. This can explain why many OEM and aftermarket radios currently in the market exhibit an inability to recognize co-channel interference during normal operations like FM Autoseek and FM station list update.
FM Autoseek involves jumping to frequencies in the spectrum starting from a presently tuned-in station until a station is reached that meets criteria for acceptable field strength. FM Station list is a feature where the tuner scans the entire band and populates a station list with stations that meet the quality criteria, such as field strength. Co-channel interference negatively affects the listening audio quality experience by radio head unit end users, especially during FM radio operations, such as FM Station List and FM Autoseek operations.
A model FM MPX signal is shown in FIG. 1. Here it is noted that the stereo signal consists of signals bandlimited to 15 kHz. The FM MPX signal is backward compatible with FM mono receivers such that the mono receivers will only utilize the baseband (L+R). Stereo FM decoders however utilize both the baseband (L+R) and the (L−R) components which are double side band modulated about the 38 kHz carrier. For example, in Europe the frequency deviation is typically +/−22.5 kHz. When combined with the modulated audio (up to ±15 kHz), this translates to 37.5 kHz peak deviation, or 75 kHz (37.5 kHz doubled) when considering both negative and positive modulation about the carrier frequency, which is in accordance with the 100 kHz frequency step in European countries. Likewise, in the United States where the deviation is +/−75 kHz, this adds up to 75 kHz+15 kHz=90 kHz peak deviation, or 180 kHz (90 kHz doubled) when considering both negative and positive modulation about the carrier frequency, which is in accordance with the 200 kHz spacing between stations in the United States.
When a radio head unit is tuned to a frequency having co-channel interference, the FM MPX signal is tainted due to having two separate signals riding on each other, i.e., superimposed on each other. Each of the two signals has a separate audio content, which results in the decoder misconstruing the signal as having severe multipath and ultrasonic noise. However, the characteristic phenomenon that arises both in the laboratory and in the field is that co-channel interference results in high field strength (typically above 40 dBuV), but the signal is tainted in that the signal has high multipath and/or ultrasonic noise due to the destruction of the FM MPX signal.
The above-described co-channel interference scenario is different from normal cases of multipath wherein the field strength varies in accordance with the multipath's destructive and constructive effects. In the case of co-channel interference, the field strength experiences constructive superimposition and the signal exhibits high multipath activity since the FM MPX signal is “destroyed”.
Accordingly, what is neither anticipated nor obvious in view of the prior art is a method of avoiding tuning into an FM frequency on which co-channel interference is present.